A two-dimensional digital image, such as a photograph, is generally exploited in the form of a matrix composed of pixels. Each pixel forms an elementary unit of the image, transmitting a piece of color information, and the image is created by the spatial organization of various colors.
In red-green-blue (RGB) color spaces such as for example the standard red-green-blue (sRGB) space commonly employed in sensors and displays, the color of a pixel is determined by the proportion of each of the three primary colors red, green and blue forming the components of each pixel.
The sRGB color space is practical from a hardware point of view but very poorly representative of the human perception of a color. In contrast, the hue-saturation-brightness (HSB) color space (also called the hue-saturation-value or HSV color space) is for its part very close to the human perception of a color. Specifically, it is natural for a person to identify a hue (colors of the rainbow), a saturation (soft, pastel or strong character) and a brightness (dark or light character).
Other color spaces and methods for converting a digital image from one color space to another do exist and are well known.
It is commonplace, for example when altering photographs, to seek to increase the saturation of an image in order to embellish its appearance.
However, increasing the saturation of certain digital images, for example by applying an identical gain to the saturation of all the pixels of the image, may rapidly denature the content of the image. For example, the colors of the human skin turn unpleasant red-orangey hues, landscapes appear artificial, soft colors perceived as almost grey become bright and the saturation modulations of highly saturated areas of uniform hue disappear.
Solutions do exist that allow certain of these adverse effects to be avoided, for example by parameterizing the saturation gain depending on the input color of the pixel according to a transfer function that, for example, will generally allow the increase in the saturation of common skin hues to be limited, or by limiting the increase in the saturation of colors that are initially already very saturated.
However, the parameterization of such solutions remains the same whatever the image and may be unsuitable for certain image contents. Typically, the saturation of the skin hues of an image containing no faces or skin will also be limited. Furthermore, since skin hues are relatively numerous and varied, general methods are often not precise enough and therefore not very effective.
Currently, human intervention is therefore required if saturation is to be increased in a way that does not denature the content of an image.
Therefore, it is either very risky to embellish the appearance of an image by increasing its saturation if it is done in real time in a digital-image processing chain such as, for example, an image signal processor (ISP) integrated into a video camera, or, in contrast, any increase is made almost imperceptible by precaution.
For this reason, ways of implementing a method allowing the appearance of an image to be improved in terms of the perception of an increase in saturation are advantageously provided. The method is dynamically adaptable to each processed image and thus has a low risk of denaturing the image while nonetheless having a visible effect.